1) Field of the Invention
The present invention relates to a technology for performing mobile internet-protocol (IP) communication on a network in which a plurality of IPs coexist.
2) Description of the Related Art
IP version 4 (IPv4) is a protocol of a network layer, and is used for packet communication in the Internet. IP addresses of IPv4 are allocated to IP communication terminals that are connected to the networks that follow IPv4. The IP address is used to identify each communication terminal and specify a position of the IP communication terminal. In other words, if one IP communication terminal wants to perform communication with other IP communication terminal connected to the network, the one IP communication terminal must know an IP address of the other IP communication terminal.
If the other IP communication terminal is a fixed communication terminal, i.e., an IP communication terminal that is always connected to a network through the same router (network repeater), a problem does not occur; because, the IP address of the other IP communication terminal is known and the IP address does not change. However, if the other IP communication terminal is a mobile communication, i.e., an IP communication terminal that is connected to a network through different routers each time the other IP communication terminal is connected to the network, its IP address changes each time it is connected to the network, and it becomes necessary to identify its IP address each time.
Conventionally, a mobile IP communication terminal performs packet communication by the following method. FIG. 10 is a schematic of an IPv4 mobile communication terminal 702 in an IPv4 network 700. It is assumed that the IPv4 mobile communication terminal 702 performs communications with an IPv4 communication terminal 703 via the IPv4 network 700. The mobile communication terminal 702 is shown to be connected to the IPv4 network 700 through a router 704b; however, the IPv4 mobile communication terminal 702 can be moved, as shown in FIG. 10 with an arrow, and connected to the IPv4 network 700 through a router 704c. 
When the IPv4 mobile-communication terminal 702 is connected to the IPv4 network 700, the IPv4 mobile-communication terminal 702 acquires Care of Address (hereinafter, “CoA”) from the IPv4 network 700. The CoA is used as a temporary IP address and indicates connection position of the IPv4 mobile-communication terminal 702. The IPv4 mobile-communication terminal 702 edits a registration information signal to register the CoA and transmits the registration information signal to the IPv4 home agent 701 via the IPv4 network 700. The IPv4 mobile-communication terminal 702 sets Home Address (hereinafter, “HoA”) in the IPv4 home agent 701 as a fixed IP address, and notifies the IPv4 communication terminal 703 of the HoA as a receiving address. As a result, when the IPv4 communication terminal 703 transmits a packet to the HoA, the packet is always received by the IPv4 home agent 701. Then, the IPv4 home agent 701 transfers the packet to the registered CoA, i.e., to the IPv4 mobile communication terminal 702.
FIG. 11 is a schematic for explaining encapsulation of a packet when the packet is transmitted to a mobile communication terminal through an IPv4 network, FIG. 12A is a schematic for explaining a packet when mobile communication is performed through an IPv4 network, and FIG. 12B is a schematic for explaining encapsulated packet when mobile communication is performed through an IPv4 network.
As shown in FIG. 11, a packet is transferred based on an HoA-CoA correspondence table 800 that is stored in the IPv4 home agent 701. A packet on a route R801 between the IPv4 communication terminal 703 and the IPv4 home agent 701 has a configuration as a packet model 820 shown in FIG. 12A. The packet model 820 includes a header 821, which has a destination address and a source address, and a payload 822 corresponding to user's data. A packet on a route R802 between the IPv4 home agent 701 and the IPv4 mobile-communication terminal 702 has a configuration as a packet model 830 shown in FIG. 12B. The packet model 830 includes the packet model 820 and a header 831. The header 831 includes CoA of the destination terminal as a destination address and the address of HAv4 as a source address. Such a process of encapsulating one packet, i.e., the packet model 820, in other packet, i.e., the packet model 830, is called packet encapsulation. When the packet model 830 arrives at the IPv4 mobile-communication terminal 702, the packet model 830 is decapsulated to obtain the packet model 820.
IPv4 has a 32-bit address; however, recently the address is thought to be too short to express required information. As a countermeasure, an IP version 6 (IPv6), which has a longer address, i.e., a 128-bit address, has been developed.
However, it is physically impossible, or, it is impossible under circumstances of users, to shift all the IP environments from IPv4 to IPv6; therefore, currently both IPv4 and IPv6 coexist. Therefore, if two or more protocols coexist in a network, it becomes necessary to switch protocols in such a manner that the communication can be performed seamlessly. Japanese Unexamined Patent Publication No. 2003-18185 discloses a method for this purpose.
It has been anticipated that IPv6 is going to be the main trend in the future, and that an IPv4 mobile-communication terminal may not be used in several cases. For example, an IPv4 mobile communication terminal cannot be used in a network that supports only IPv6. Moreover, an IPv4 mobile-communication terminal can not be used if an IPv4 address for CoA of a mobile-communication terminal cannot be acquired in a network even though the network supports both IPv4 and IPv6, and if it is impossible to perform packet communication with an external network by IPv4 in a network.